This invention relates generally to communication systems including modulation techniques and in particular to a system and method for communicating a signal over a noisy communications link/channel. In more detail, the invention relates to the transparent communication of analog and digital signals over noisy channels. The system and method may be used for communicating various types of data over various different communication channels. For example, the system may be used to provide fast Internet access over wireless links.
As the demand for high-speed Internet access grows, the existing solutions fall short of expectations in many situations. One access solution that has become popular are cable modems. The cable TV infrastructure is used for providing Internet access by allocating one or more channels of the cable for that purpose. The internet access channel is then a shared medium used by multiple subscribers. Each subscriber has a cable modem that communicates with the cable facility head-end via the cable channel and a specially allocated return channel, usually in the same cable. Several protocols have been devised for controlling the access over this shared medium, including the DOCSIS protocol that is gaining popularity in the USA. Cable modems are becoming mass-market commodities with low unit prices and compatibility with head-end cable modem termination systems (CMTS) of multiple vendors. Another high-speed Internet access option is a family of digital subscriber line (DSL) modems operating over telephone line twisted pairs. DSL is quickly becoming a low-cost access method. Yet another option is fiber optics that allows very high speed access.
All of the above current access methods, however, have significant drawbacks. For example, fiber optics, which is the fastest access technology, is too expensive to reach each home or office and is therefore unavailable for many people. The cable infrastructure is not available in commercial areas and some cable installations are not equipped with a return channel. Finally, DSL has a limited range and speed and as the access bit rates required increase, the DSL range is further reduced.
An alternative to the above methods is fixed wireless access. Wireless access can take the form of point to point, point to multipoint or multipoint to multipoint communications. By using directional antennas and millimeter wave frequency bands where commercially licensed spectrum is more abundant than at lower frequencies, it is possible to provide broadband access services to a large number of subscribers over line of sight wireless links. These wireless links, despite their own limitations of a line of sight, rain fading and high-cost, are the preferred choice where fiber, cable or DSL links cannot provide the desired cost/performance objectives.
As wireless access products emerge, their economic limitations are becoming more apparent. One such limitation is the cost of the infrastructure. In particular, to deploy a complete access network, the infrastructure must include base stations, switching equipment and backbone links. The wireless network is not necessarily more expensive than a cable-based network, however, the up-front investment in such a network is a major burden to the service providers. Another limitation is the cost of the subscriber's equipment. This equipment is expensive partially due to the millimeter wave components and partially due to the relatively low-volume production of the equipment. Finally, as the wireless access standards evolve, a variety of protocols and equipment types are being introduced that cause a fragmented market and limits the mass market cost-reduction opportunities for any one piece of equipment so that the equipment costs remain high.
To overcome such cost limitations, it is desirable to provide wireless solutions that take advantage of the existing infrastructure available for cable or DSL modem, thus reducing the up front capital investment required for installing wireless access services. If properly implemented, such solutions could share the same cable facilities and hub equipment that is already installed and provide land-based services. Furthermore, it is desirable to be able to use existing cable and DSL modems as the subscriber interfaces for a wireless access system, thus taking advantage of these low-cost mass-market products. It is also desired to accommodate a variety of evolving subscriber equipment in a wireless network with minimal changes to the wireless equipment.
Such desired flexibility requires a transparent wireless link that allows the transmission of any band-limited signal. This equipment could operate as a wireless repeater, which is essentially an up converter to the microwave frequency at the transmitter, and a down converter at the receiver. While this approach is used at low frequencies, it is not sufficient in most wireless applications. As an example, cable modem modulation is typically 256QAM (256 quadrature amplitude modulation) or 64QAM. However, most wireless links only tolerate effectively more robust modulation schemes, such as 16QAM and 4QAM, as those links produce too much interference and phase noise to tolerate higher modulation levels. If the cable modem or DSL equipment should remain unchanged, the wireless repeater must convert the high-modulation of the subscriber equipment to a lower modulation mode. Such conversion is feasible but has several limitations. The demodulation requires decoding of the forward error correction (FEC) overhead to overcome any errors in the cable section from the CMTS to the wireless equipment. This decoding causes a significant delay if interleaving is used. Finally, such scheme is not transparent and changes to protocol require different demodulation-remodulation equipment. It is therefore desirable to be able to pass such modulation transparently to the subscriber equipment modulation scheme and yet obtain the robustness for the wireless link. These seemingly contradicting requirements are feasible in principle if the wireless link is trading off bandwidth for robustness so that it is acceptable to sacrifice reasonable amount of bandwidth to obtain the robustness. It is highly desirable that the excess bandwidth will not be essentially higher than Shannon bound for channel capacity as is well known from Information Theory.
A general approach for maintaining transparency is to treat the cable or DSL modem output as band-limited analog information. In particular, although the modem transmission carries digital information, its quadrature amplitude modulated (QAM) output does resemble band-limited Gaussian noise. If the radio link is modeled as a white Gaussian channel, there are several well-known approaches to transmit those modem signals over that radio link. One such technique is analog modulation. If the radio link bandwidth is increased, the output signal to noise ratio (SNR) can also increase. For example, a modem signal using 256QAM modulation could use a frequency modulated (FM) link with sufficient bandwidth expansion such that a radio link SNR of 20 dB will result in an output SNR of 40 dB in the FM link output that is sufficient for 256QAM. The analog modulation approach has a major drawback in the form of a threshold effect that reduces output SNR significantly at lower link SNR so that the overall radio link margin is much lower than a digital link of the same bandwidth and throughput.
A digital alternative to transmit the modem signal over a noisy link also exists. It is based on the well-known pulse code modulation (PCM) technique. The modem output, treated as an analog channel, is sampled, converted to digital, compressed using source-coding techniques and transmitted over a channel using a combination of QAM and FEC. It is also well known that if the source is a band limited gaussian process, then it is possible with proper compression and error-coding technique to closely approximate the channel capacity bound based on Shannon's theory. PCM, however, has a key drawback for the transparent modem transmission application considered here. In particular, to make efficient use of bandwidth, the PCM encoder distorts the signal to provide the lowest acceptable level of SNR_in for the payload information. This means that a compressed link will always perform at that level of distortion, even when the actual link SNR is well above threshold conditions. Alternatively, one can allow lower distortion, which is better for the modem link, but requires higher link system gain. It is highly desirable to maintain the simplicity of an analog bandwidth expansion channel and the coding efficiency of a digital link, to allow bandwidth efficiency and allow operation with increasing performance margins when the link SNR is above minimum, without increasing the system gain requirements. It is to this end that the invention is directed as described below in more detail. However, the system and method for combining an analog signal and a digital signal in accordance with the invention may also be used for other communications links. In addition, the system and method in accordance with the invention may also be used for other signal recording, such as recording data onto a compact disk or other media, and signal processing applications.